A continuous hybrid cell line, formed from a mouse neuroblastoma and a rat glioma, has been shown to form functional synapses with normal mouse myotubes in culture. These cells can be maintained in continuous logarithmic growth or can be caused to differentiate by treatment with dibutyryl cyclic adenosine monophosphate (db cAMP) or X-irradiation. The cells are electrically responsive to serotonin and acetylcholine. Receptors for a number of neuroactive compounds including narcotics have been demonstrated by others. They represent an attractive model system for structure of synapse development and modulation. Fundamentally different mechanisms are involved in the action potentials (A.P.) generalized by different types of mouse neurons in culture. Dorsal root ganglion cells exhibit both Ca ions dependent and Na ion dependent components; this latter component is largely tetrodotoxin (TTX) insensitive. Spinal cord neurons, by contrast, are exclusively due to Na currents and these are completely TTX sensitive.